1. Field of the Invention
The present invention relates to a magnetoresistive effect element and a magnetoresistive random access memory using the same and, more particularly, to a spin transfer torque writing type magnetoresistive effect element.
2. Description of the Related Art
Recently, many solid-state memories that record information on the basis of new principles have been proposed. Of these solid-state memories, a magnetoresistive random access memory (MRAM) using the tunnel magnetoresistive (TMR) effect is known as a solid-state magnetic memory. The MRAM uses a magnetoresistive effect element (TMR element) that achieves the tunneling magnetoresistive effect as a memory cell, and the memory cell stores information in accordance with the magnetization configuration of the TMR element.
The TMR element includes a magnetization variable layer and magnetization fixed layer. The resistance is low when the magnetization direction in the magnetization variable layer is parallel to that in the magnetization fixed layer, and high when the magnetization direction in the former layer is antiparallel to that in the latter layer. Information is stored by using this difference between the resistance states due to the magnetization configurations of the magnetization variable layer and the magnetization referenced layer.
A so-called current-induced magnetic field writing method is known as a method of writing information in the TMR element. This method changes the magnetization configuration of the TMR element by a magnetic field generated by an electric current flowing through an interconnection formed near the TMR element. If the size of the TMR element is decreased for high-density MRAMs, a coercive force Hc of the magnetization variable layer of the TMR element increases. In the magnetic field write type MRAM, therefore, an electric current in a write operation often increases as the size of the TMR element is decreased. This makes it difficult to achieve a small cell size for obtaining a large capacity exceeding 256 Mbits and a low writing electric current at the same time.
As a writing method of solving this problem, the spin transfer torque writing method using spin momentum transfer torque (STT) has been proposed (U.S. Pat. No. 6,256,233). This spin transfer torque writing method changes (reverses) the magnetization configuration of the TMR element by supplying an electric current perpendicularly to a direction in which films forming the TMR element oppose each other.
An electric current Ic necessary for spin transfer torque magnetization reversal is often defined by a current density Jc. Accordingly, as the area of a plane through which the electric current passes in the TMR element decreases, the injection electric current Ic for reversing magnetization also decreases. When writing information at a constant current density, the electric current Ic decreases as the TMR element size decreases. Therefore, the spin transfer torque writing method is in principle superior in scalability to the magnetic field write method.
Unfortunately, the following problem arises when implementing an MRAM by using the spin transfer torque writing method. That is, the electric current required for magnetization reversal is presently larger than an electric current value that can be generated by a selection transistor often used in the implementation of an MRAM. This practically makes it impossible to allow a device to operate as a memory by using the spin transfer torque writing method.
As described in C. Slonczewski, “Current-driven excitation of magnetic multilayers”, “JOURNAL OF MAGNETISM AND MAGNETIC MATERIALS”, 1996, vol. 159, pp. L1-L7, a reversing current for magnetization reversal by spin transfer torque effect generally depends upon saturation magnetization Ms of a magnetization free layer. Accordingly, it is important to decrease the saturation magnetization Ms for the magnetic switching in the magnetization free layer by spin transfer torque effect of a low electric current. When the saturation magnetization is decreased by using the present techniques, however, the thermal stability required for the data retention characteristics of a non-volatile memory also decreases.